This invention relates generally to apparatus for heating particulate material and, more particularly, for heating particulate material which is used on roadways. The apparatus is of the type having an elongated drum adapted to be rotated about its own axis and having a burner at one end of the drum for producing a hot gaseous flame in the drum. Particulate material is introduced into the drum and is advanced from one end of the drum to the other with a tumbling action as the drum is rotated. During such advance, the material is heated by the stream of hot gases from the burner flame.
The particulate material may, for example, be virgin aggregate which is mixed with a binder such as liquid asphalt to produce a bituminous paving material. The aggregate is heated and dried in the drum by the hot gaseous stream and is mixed with the asphalt binder as the aggregate tumbles through the drum. Alternatively, the binder may be added to and mixed with the dried aggregate in a pugmill after the aggregate has been discharged from the drum.
Apparatus incorporating the principles of the invention also may be used to recycle old asphaltic paving material in order to prepare a new mix. In such an instance, the old asphalt is broken up into particulate form, is delivered into the drum, and is rendered plastic and workable by the heat in the drum. Usually, a liquid rejuvenating or softening agent is mixed with the old asphalt, the mixing either taking place in the drum itself or taking place in a pugmill immediately after the asphalt has been discharged from the drum.
It has been recognized that it is desirable to avoid direct exposure of the particulate material to the burner flame or to the hottest portion of the gaseous stream, regardless of whether the particulate material is virgin aggregate or is recycled asphalt. The various problems resulting from such direct exposure are discussed in detail, for example, in Brown U.S. Pat. No. 4,130,364; Schlarmann U.S. Pat. No. 4,165,184; Malbrunot U.S. Pat. No. 4,300,837; Schlarmann U.S. Pat. No. 4,318,619 and Malipier et al U.S. Pat. No. 4,318,620.
It also has been recognized that control must be maintained over pollution of the atmosphere caused by the emission of "fines" from the aggregate or the recycled asphalt. In order to meet the clean air regulations of the Environmental Protection Agency (EPA), many drum-type units must employ an expensive dust collector or precipitator in association with the exhaust stack of the unit. Units which attempt to maintain a "clean" exhaust without the use of a dust collector or the like are disclosed in Shearer U.S. Pat. No. 3,832,201; Shearer U.S. Pat. No. 4,025,057; Benson U.S. Pat. No. 4,229,109 and Graham U.S. Pat. No. 4,249,890.